This invention relates to an integrated circuit relaxation oscillator that is temperature compensated and capable of operating at a low supply voltage.
An oscillator providing those features has a capacitor external to the integrated circuit that is charged by a current source in the integrated circuit, and has an external resistor paralleling the external capacitor which resistor discharges the capacitor at a rate much slower than the charge rate so that the frequency of oscillation and its stability with temperature are dependent essentially only upon those two external components and the trip point levels provided by the integrated circuit.
A known circuit of this kind is shown in FIG. 1. When the supply voltage +Vcc is first applied to terminal 10 the voltage across this external capacitor 12 is zero, the voltage at terminal 14 is at +Vcc, transistor 16 is turned on and transistor 18 is held off. Transistor 20 is also held off because its base voltage is less than +Vcc which appears at the base of transistor 16. Transistor 22 is turned on establishing a current through resistors 24 and 26 that reduces the voltage at the base of transistor 20. Transistor 28 is also turned on producing a charging current 45 in the external capacitor 12. Also at that time, transistor 32 turns on and in turn activates a series of PNP current source transistors 33, 34 and 35 that constitute the oscillator load.
This particular oscillator, in fact, is in an FM-radio stereo-decoder. It oscillates at 76 KHz, synchronized with a 19 KHz stereo pilot signal being connected via transistors 37 and 34 in a phase locked loop (not shown). Frequency dividers (not shown) are driven by the PNP transistors 33, 34 and 35, one of which is in the loop.
When the voltage at terminal 14 falls to just below the base voltage of transistor 20, then it turns on and transistor 16 turns off. Transistor 28 turns off also and the charging current 45 drops to zero. Thus, the trip point for terminating the charging period is determined by the sum of the resistor values of resistors 24 and 26 and the value of the current supplied by current sources 40 and 42.
Current source 40 has a positive temperature coefficient while that of current source 42 is negative. The total current is split, a quarter of it passing through the 1X base-emitter junction of transistor 22, another quarter of it passing through the 1X transistor 32 and half of the total current (current 45) passing through the 2X transistor 28 to charge the capacitor 12.
The capacitor discharges through external resistor 44 causing the voltage at terminal 14 to rise. Meanwhile, transistors 22, 28 and 32 are off and the 4X transistor 18 is on producing a current in resistor 24 that amounts to all of the current from the current sources 40 and 42. This produces a drop across resistor 24, a small value resistor, which sets the voltage at the base of transistor 20 higher than it was during the charge period.
When the voltage at terminal 14 equals and just exceeds the voltage at the base of transistor 20, then transistor 20 turns off, transistor 16 turns on and a new charge period begins. Thus, the termination of the discharge period is determined by the value of the current produced by current sources 40 and 42 and by the value of resistor 24.
The relaxation oscillator circuit, therefore, has the advantage that the trip point voltage levels are determined essentially only by the values of the current from current sources 40 and 42 and of the resistors 24 and 26. Furthermore, this all-NPN-transistor oscillator is operational for Vcc levels as low as 1.8 volts.
However, it has been discovered that such integrated circuit oscillators tend to lock up. This apparently stems from the need to make the currents in transistors 22, 38 and 32 much larger than the discharge current through resistor 44, so that the capacitor charge rate will be faster than the discharge rate to minimize the secondary factors determining oscillation frequency and frequency dependence upon temperature. In this situation only a small fraction of the charge current 45 is enough to equal the discharge current. That charge-discharge current equality condition can freeze the voltage across the capacitor 12 and at pad 14 at the end of a discharge period when transistor 16 is turned on slightly at a base voltage insufficient to fully switch it on.
It is, therefore, an object of this invention to provide a temperature compensated relaxation oscillator that is not subject to locking up.
It is a further object of this invention to provide such an oscillator that operates at a low power supply voltage.